Recently, a social demand for weight saving of vehicles such as motorcars has increased more and more out of consideration for the global environment. To meet such a social demand, aluminum alloy materials are investigated in place of steel materials such as steel sheets as materials for auto panels, particularly large body panels (outer panels and inner panels) such as panels for a hood, a door, and a roof.
The Al—Mg (5000-series aluminum) alloy sheet (hereinafter, sometimes referred to as Al—Mg alloy sheet) including JIS 5052 alloy and JIS 5182 alloy has high ductility and strength, and therefore has been used as a material for forming (press forming) for such large body panels.
However, as disclosed in PTL 1 and the like, when such an Al—Mg alloy sheet is subjected to a tensile test, yield elongation may occur in the vicinity of the yield point on a stress-strain curve, or saw-toothed or stepwise serrations may occur on the stress-strain curve at a relatively large amount of strain (for example, tensile elongation of 2% or more) beyond the yield point. Such phenomena on the stress-strain curve cause so-called stretcher strain (hereinafter, sometimes represented as SS mark), leading to a significant problem that reduces commercial value of the large body panel as a forming product, particularly of the outer panel the appearance of which is a commercially important factor.
As generally known, the SS mark is classified into two types, i.e., a so-called random mark as an irregular beltlike pattern such as a flame pattern formed in a region of a relatively small amount of strain, and a parallel band as a parallel beltlike pattern formed so as to define about 50° with respect to a tension direction in a region of a relatively large amount of strain. It is known that the former (a first type) random mark is caused by yield point elongation, and the latter (a second type) parallel band is caused by the serrations on the stress-strain curve.
There have been provided various methods for preventing such types of SS mark. For example, as a main approach, it has been known that particles of the Al—Mg alloy sheet are controllably coarsened to a certain degree. However, such an approach of particle control is not effective for preventing formation of the parallel band as the second type of the SS mark. If the particles are excessively coarsened, another problem such as surface roughening is rather caused during press forming.
As another approach for preventing the SS mark, it has also been known that a refined material of the Al—Mg alloy sheet is subjected to working (pre-working) such as skin-pass or leveling before being press-formed into the large body panel so that a slight strain (pre-strain) is added thereto. Even in such a pre-working approach, if the material is too highly worked, the serrations on the stress-strain curve are likely to occur, easily leading to formation of a wide and clear parallel band during actual press forming.
In contrast, PTL 1 provides a method of manufacturing the Al—Mg alloy sheet, in which formation of both the random mark and the wide parallel band is suppressed. In such a method, a rolled sheet of the Al—Mg alloy is subjected to solution treatment and hardening treatment, and is then subjected to cold working as pre-working followed by final annealing, and thereby a sheet with an average particle size of 55 μm or less and without coarse particles is produced.
PTL 2, which makes no direct description on suppression of SS mark formation, describes that a heating curve from room temperature is obtained through measurement of thermal variation of an alloy sheet by differential scanning calorimetry (DSC), and a position and a height of an endothermic peak on the heating curve are used as guidelines for improving press formability of the alloy sheet.
However, a demand level for a surface texture becomes strict more and more in a recent large body panel, particularly in an outer panel the appearance of which is a commercially important factor. In each of PTLs 1 and 2, the measure to suppress the SS mark formation is not enough to meet such a demand.
In contrast, as exemplified in PTL 3, there is provided a technique in which 0.1 to 4.0% of Zn is particularly contained in the Al—Mg alloy sheet, and thereby the amount of clusters (ultrafine intermetallic compounds) formed by Al and Mg is increased as clusters that each further include Zn, so that the critical strain amount (limit strain amount) for serrations is increased, and the effect of increasing the limit strain amount is further enhanced. It is described that this makes it possible to suppress formation of both the random mark and the parallel band, and it is possible to produce an Al—Mg alloy sheet that is suppressed in SS mark formation and good in formability such as press formability into an auto panel.
PTL 4 defines an average particle diameter in particle size distribution determined by a small-angle X-ray scattering method and average number density of peak sizes in the particle size distribution, as guidelines for indicating a relationship between the microstructure of an Al—Mg alloy sheet that also contains Zn and press formability represented by the SS mark or the like.
However, when the Al—Mg alloy sheet contains a large amount of Zn, another issue arises, i.e., age hardening at room temperature tends to occur. This is because while PTL 3 describes the clusters (ultrafine intermetallic compounds) including Zn as the best measure to suppress SS mark formation, such clusters are easily formed at room temperature.
In general, the Al—Mg alloy sheet is not formed into a product such as a large body panel by an automaker immediately after being manufactured by an aluminum sheet manufacturer, but is formed into the product some weeks later after that. Hence, for example, when the Al—Mg alloy sheet is formed into a product such as a large body panel after the lapse of one month from manufacturing of the sheet, age hardening proceeds, and a new (another) issue arises, i.e., bendability or press formability is rather degraded.
As generally known, age hardening at room temperature is in general less likely to occur in the Al—Mg alloy sheet compared with a heat-treated Al—Zn—Mg (7000-series) alloy sheet. However, when such an Al—Mg alloy sheet has a high content of Zn as in PTL 3, the sheet also shows age hardening at room temperature as with the 7000-series aluminum alloy sheet.
In contrast, PTLs 5 and 6 each devise a technique in which Cu is contained in the Al—Mg alloy sheet as an element effective for suppressing SS mark formation, in place of Zn that tends to cause age hardening at room temperature. However, even if an Al—Mg alloy sheet contains Cu, the sheet may not exhibit the effect of suppressing SS mark formation. Specifically, a formation state of the SS mark is greatly affected by an existing state (a microstructural state) of Cu in the Al—Mg alloy sheet.
In PTL 5, therefore, the microstructure of the sheet is indirectly defined by the endothermic peaks between 180 and 280° C. on a heating curve (DSC heating curve) from room temperature determined by differential thermal analysis (DSC).
In PTL 6, the microstructure of the sheet is more directly defined by average density of clusters including Cu atoms, each Cu atom being in specific connection with other Cu atoms adjacent thereto, in atomic clusters determined by a three-dimensional atom probe field ion microscope.